The invention relates to transistor amplifiers and has particular application to complementary symmetry amplifiers. In a typical complementary symmetry amplifier, complementary transistors have their emitters coupled together and their collectors coupled to the two terminals of a power supply. The PNP transistor collector is coupled to the negative supply terminal and the collector of the NPN transistor is coupled to the positive power supply terminal. Desirably the load device which couples to the emitters is returned to a potential midway between the positive and negative terminals and typically this is referenced as ground potential. This configuration avoids the large coupling capacitor that must be used if the load is returned to any other potential. If the transistor base terminals are coupled together and driven from a common driver amplifier, the output will develop through emitter follower action to the load device. Thus the circuit is a voltage follower with the NPN transistor responding to the positive voltage excursions and the PNP responding to the negative excursions.
If only switching is desired this simple basic circuit works very well. However, if it is to be used as a linear amplifier it suffers from what is called crossover distortion. It can be seen that with no input signal neither transistor will conduct because the bases are at emitter potential. No conduction will occur until the transistor threshold is exceeded (about 0.6 volt in silicon devices operating at room temperature). Thus signals smaller than about 1.2 volts peak-to-peak will not be amplified appreciably. For an audio signal, this characteristic results in severe distortion that increases with smaller signals.
To avoid crossover distortion is common to provide a small bias selected to just barely turn the two transistors on with no applied signal. As a practical matter, the greater the zero signal current, the lower the crossover distortion. Therefore, ideally, the transistors are fully biased into class A operation for lowest distortion. However, this results in constant power supply drain which means that it is just as economical to use a single class A biased transistor. It is conventional to employ sufficient bias on the transistors so that they operate class A for very small signals and class B for large signals so that low quiescent current drain is combined with high power capability with tolerable distortion at all levels.
Unfortunately the actual bias selected will be a matter of judgement when weighing distortion against quiescent power economy. In any event some means of developing a quiescent bias is required and this has been the subject of much circuit development work. Since the bias voltage must ordinarily be varied as a function of transistor junction temperature, many rather complicated circuits have evolved.